The present invention relates to a light emitting diode lighting system and more particularly to a light emitting diode array, which is used, for example, in machine vision identification and systems used to inspect production objects such as mechanical components, printed circuit boards, foodstuffs, and the like.
In the field of vision identification and inspection, through the use of a lighting system, a well-defined lighting system results in many benefits. Some of the benefits include, but are not limited to, simplification of the computer algorithms necessary for reliable object identification and reduction of erroneously rejected objects. There are benefits with minimizing the space, cost and maintenance of the lighting system. Well-defined lighting systems result in efficiencies during operation and manufacture.
Light emitting diode (LED) array light sources are widespread in use in a variety of different signaling and lighting applications. Typical uses of LED arrays include image sensors, inspection of parts, luminaries, and the like. It is generally advantageous to connect all of the LEDs in series because the result is a high-voltage and low-current load that is more economical than low-voltage and high-current connection. However, the high-voltage and low-current approach has a problem. For example, if one of the LEDs that are connected in series fails by an open-circuit condition then the rest of the LEDs connected in series will not operate. Consequently, LED arrays incorporate a combination of series connected and parallel connected strings of LEDs to avoid failure of the entire array. However, this solution is complex to manufacture, not as economical as an all series LED array, and creates an array with a fluctuating light output.
In one prior art application a light emitting diode (LED) array connected in series has an active shunt arrangement for sensing a failure of one or more of the LEDs, and for shunting current that would have otherwise flowed through a failed LED. This scenario maintains a flow of current through the remaining LEDs. The active shunt arrangement includes several active shunts connected in parallel across respective ones of the LEDs, and remote sense and digital logic for detecting an open-circuit condition of the normally closed circuit, and for sequentially activating the active shunts until the normally closed circuit has been restored to a closed-circuit condition. However, a problem associated with this arrangement is that it requires a complex arrangement of circuits and controls that require an individual circuit to sense a failure condition of an associated LED. Each active shunt is an active switching device connected in parallel with each LED that consumes more energy than a simple LED array connected in series, and produces fluctuations in the light output when the shunts are engaged.
The stability of the light field is important wherever thresholds need to be set, for example, in software, as part of the inspection criteria. Stability and efficiencies can be achieved by regulation of the power delivered to the light source, internal controls within the light source, or auto-feedback of the light field back to the camera or light source. What is needed are improved internal controls within the light source to regulate light level output, regulate the relative operating temperature and indicate the LED head forward voltage.
It is an aspect of the invention to regulate the temperature of LED arrays that increases the efficiency of the forward current.
It is another aspect of this invention to control the level of light output of LED arrays and compensate for efficiency altering effects on LED arrays.
A light emitting diode lighting apparatus includes a power supply with a fixed direct current, a light emitting diode head for emitting light, and a controller for adjusting the level of light to compensate for efficiency altering effects. The controller receives from the light emitting diode head signals for optical feedback stabilization, temperature compensation, and detection of short-term current changes to adjust said light and efficiency.
A light emitting diode (LED) head includes a body that is internally hollow for positioning a light emitting diode cluster inside and connected with a signal cable. The light emitting diode cluster is mounted on a platform for emitting a desired level of light. A photosensitive device that collects a representative amount of light from one or a number of LEDs, using the photocurrent generated as a regulation parameter signal to drive the LED cluster through an intelligent controller, is mounted on the platform but optically isolated from the LED cluster. Also mounted on the platform is a thermistor for monitoring temperature of the LED cluster, by generating an operating temperature signal to drive the LED cluster through an intelligent controller, with the thermistor located within the LED cluster. Furthermore, a LED failure detector that continuously monitors the current of the LED cluster is mounted on the platform, allows the detection of short term current changes, and provides a signal to drive the LED cluster through an intelligent controller. The system monitors head voltage and indicates a change in the head voltage due to LED failure. The platform is mounted inside the body and a plate and retaining ring secure the platform inside the body.